1. Field of the Invention
The invention relates to a probe for measuring thin layers on a base material using a magnetic or eddy current process.
2. Background Art
In nondestructive measurement of the thickness of solid layers such as enamels, galvanic layers and the like, on generally metallic base materials, or in the measurement of the thickness of films such as plastic films which have been applied to a metallic base material, layer thickness measuring instruments which exploit a magnetic or eddy current process are used. In these measuring instruments the measurement pole of the measurement sensor is made spherical with a radius of curvature of typically 1 to 10 mm. The measurement sensor with the spherical measurement pole is placed either directly by hand on the surface of the layer to be measured, or the measurement pole is under the action of a pretensioned spring which is located in the holder of the measurement sensor, with a specific load pressure on the layer to be measured. When the measurement probe is placed directly, the load pressure depends on the weight of the hand and can be from a few tenths of a Newton to a few dozen Newtons.
When measuring solid layers the differing load pressure of the measurement pole is of secondary importance. When measuring powdery or soft layers on a solid base material or layers on an elastic base material, however, various disadvantages arise.
When the measurement pole with a pretensioned spring is placed in the probe holder, the load pressure of the measurement pole is constant and is typically 0.5 to 1 Newton.
The disadvantage of this type of probe for the measurement of powdery or soft layers, but also thin layers especially on elastic base materials is however that the load pressure when using typical radii of curvature of 1 to 10 mm of the spherical measurement pole exerts such a great compressive effect on the surface that for example a powdery or soft layer is punctured or the measurement pole penetrates at least strongly and to various depths into the layer material.
For elastic base materials deformation of the measured article can be caused by the compressive effect. The result is incorrect measured values.
This danger also exists when the measurement sensor is lowered by hand. The mass inertia of the measurement sensor leads to a considerable force acting on the layer when the measurement pole is placed on the surface, even if it weighs only a few grams, as a result of the abrupt deceleration of the hand from roughly 10 to 20 cm/s to 0 cm/s within a path length of only a few microns, so that the measurement pole can penetrate as far as the base material especially for powdery or soft layers or the layer can be indented at least to a considerable degree. The mass forces are between 0.2 to 20 Newton when the probe is placed by hand.
The mass forces during measurements act similarly on pliable materials of the measurement article.
A reproducible measurement when using typical known probes is, therefore, not possible on powdery or soft layers or on measured articles with pliable materials at these high and varied forces. Practice confirms this statement. Depending on how the probe is placed, the measured values decrease by few percent or to zero percent of the original layer thickness. The measurement result is therefore essentially useless. For this reason contact layer thickness measurement probes are not used in particular for measuring powdery and soft layers.
The same applies to measurements on articles with pliable materials.
In practice there is also the urgent desire to determine the thickness of still powdery or soft layers before firing or hardening in order to be able to make a correction as quickly as possible before the next working process.
At present the correction can be made only after measuring the solid layer, therefore after firing or hardening, since commercial measuring devices are only able to measure solid layers. The waiting time, for example, in a powder coating, between application of the powder layer and the earliest possibility of layer thickness measurement of the fired layer is roughly one hour; this is a time interval which is very uneconomical if the deviations of layer thickness must be corrected.
One example of layers to be measured are also thin enameled layers on metal foils, for example, internally enameled aluminum tubes for toothpastes and similar mixtures. Conventional hand measurement probes bend the thin base material when the measurement pole is placed and, therefore, do not deliver reproducible results.
German Published Patent Application No. 3,622,708 A1 discloses a measurement probe which is used to test final paint jobs on motor vehicle chassis. This measurement probe is gimbaled and guided lengthwise on the chassis by means of automatic handling machinery. It is placed vertically on the surface to be measured by the automatic handling devices with a certain contact pressure. By pretensioning of a compression spring which supports the measurement probe the full pretensioning force of this compression spring takes effect after contact of the probe pole with the surface to be measured. During the measurement the measurement probe is held by a suction cup on the chassis.
To lift the probe system off the chassis air must be introduced again into the suction cup via the suction line. In this way the vacuum of the suction cup is canceled, by which the suction cup is lifted off of the surface of the chassis. It is not possible to place the measurement probe on the same measurement point with pinpoint accuracy again and check the first measurement.
Another layer thickness measurement probe is known from British Patent No. 637,471. This measurement probe uses the electromagnetic adhesive force principle. The holding force of the pole pin which acts on the surface of the coated soft-magnetic steel base material derives from the interaction of an armature spring with low spring force and the electromagnetic force of the coil system which is supplied with direct current of a battery. The adhesive force is indirectly measured by the exciter current flowing through the coil. The measured quantity is the current measured on an ammeter which, by continuous reduction starting from a maximum current, is no longer enough to hold the pole pin against the compressive force of the armature spring on the layer surface. When the pole pin is removed from the surface to be measured, a contact is closed which causes a lamp to illuminate after the switch is turned on. At this instant, further reduction of the current must be stopped using two resistors since this current value is a measure of layer thickness.
Another measure of layer thickness is the spring force of the armature spring which acts as a measurement spring together with the electromagnetic adhesive force of the pole pin. Diminution of the spring force, for example, by aging, changes the deflection of the ammeter and thus also the layer thickness display. In this measurement device the slackening of the armature spring would lead to a smaller layer thickness display than that of the actual layer thickness.
In the known device there is also a second larger spring which however does not act on the pole pin and, thus, has no effect on the load pressure of the pole pin on the layer surface. In addition, the electromagnetic adhesive force in this measurement probe depends very heavily on the layer thickness, so that as a result of the measurement principle strong different load pressures must arise. A constant contact force or almost zero load pressure is not possible due to the electromagnetic adhesive force principle in this known measurement probe. If the two contacts for the display light are closed, the pole of the pole pin can be pressed with any strength against the surface by exerting variously intense pressure on the measurement probe. This known measurement probe, therefore, does not enable usable measurement results for thin, powdery or soft laminated materials.
German Published Patent Application No. 3,902,095 A1 discloses another measurement probe for measuring thin layers on electrically conductive base materials. When the probe is placed, a sliding sleeve touches the measured article. Since the probe, however, is held by hand on the outer grip sleeve, this grip sleeve slides down in the direction of the measured article in one motion. In doing so the probe body is moved down via two stops against the action of a helical spring. This helical spring, however, is designed only to press back the probe body in the rest state into the sliding sleeve or to elastically oppose the probe body with a sensor part supported in leaf springs after placing the sliding sleeve, until the spherical surface of a cup core which forms the measurement pole sits on the surface of the measured article. Softly setting the spherical surface of the measurement sensor in this known measurement probe is only possible within the framework of the stipulated small spring path of the two stationary-mounted leaf springs and, thus, only within narrow limits. In this known measurement probe, as a result of the short spring paths, relief of the spring system is not possible, by which also it becomes impossible to place the sensor system again with pinpoint accuracy for a repeated measurement process at the same location. When the grip sleeve is released, brief lifting of the entire measurement probe from the measurement surface in manual operation cannot be avoided, unless the probe were to be guided in a stand. When the probe is placed again by hand, however, another measurement point is touched in any case. In this known measurement probe the load pressure is fixed by the construction features of the probe and the leaf springs used. Adjustment of a maximum load pressure by the probe operator is not possible. Other load pressures require probes with different construction features.
The load pressure of the measurement sensor is otherwise also dependent on the measurement direction, to the extent that in measurements from top to bottom it is different than in overhead measurements from bottom to top. These difference load pressures are invariable. It is not possible to adapt the spring system depending on the measurement direction.
In the known measurement probe the measurement system is also freely elastically suspended with two leaf springs and can therefore swing freely axially. When the probe is moved between individual measurements and during all handling, however, acceleration and deceleration forces occur which allow the measurement system to continuously push against the stop surfaces inside of the probe. These pushes are disadvantageous for the measurement properties and the measurement accuracy of these precision measurement devices.
Since processes of placement of the sliding sleeve on the measured article and the measurement system with the spherical placement surface of the cup core on the surface to be measured cannot be carried out in separate steps, in this known measurement probe other disadvantages arise. The process of placing the entire probe with its mass and with the mass of the hand and arm of the operator cannot be executed in a controlled manner with respect to placement speed.
In this way the time of contact of the face of the sliding sleeve with the measured article cannot be unambiguously detected by the operator. Softly placing the cup core with minimum transfer of momentum requires immediate braking of the placement process after the first contact of the sliding sleeve, so that the measurement system very slowly touches the surface. But this is not possible in the known measurement probe due to the uncontrolled placement speed which is caused by the different masses of the probe, hand, and arm of the operator.